1. Field of the Invention
The present invention relates to a decision-feedback equalizer for cancelling continuous wave (CW) interference and intersymbol interference.
2. Description of the Related Art
A decision-feedback equalizer, as described in a paper "Rejection of CW Interference in QPSK Systems Using Decision-Feedback Filters", Loh-Ming Li et al., IEEE Transactions on Communications, pages 473-483, Vol. COM-31, No. 4, April 1983, comprises a feedforward filter for receiving an input signal for cancelling narrow-band continuous wave (CW) interference, a decision circuit, and a feedback filter for receiving the output of the decision circuit. The outputs of both filters are combined to cancel intersymbol interference and applied to the decision circuit where a decision is made between binary levels. Each filter is a transversal filter configuration in which a tapped delay line is formed and a plurality of tap-gain multipliers are connected to the delay-line taps, the outputs of the multipliers being summed to produce the filter output. The tap-gain values of the multipliers are derived from a decision error and adaptively updated according to the least-mean square (LMS) algorithm. If the narrow-band CW interference is a sequence of periodic bursts, the tap-gain values of the multipliers of both filters must be quickly adapted to the presence and absence of CW bursts. However, the exponential convergence characteristic of the LMS adaptive control causes the tap-gain values to rise exponentially from zero to optimum values at the leading edge of each CW burst and then decay exponentially to zero at the trailing edge of the burst. If the CW burst rate is high, the adaptive tap-gain control would be too slow to adapt itself to the varying level of interference and the decision-feedback equalizer would suffer undesirable performance degradation. In addition, the decision error is of substantial value when the equalizer is affected by such strong interference where the D/U (desired-to-undesired) ratio assumes a negative value. Under such circumstances, the adaptive tap-gain control would diverge due to the propagation of decision errors along the tapped delay line of the feedback filter. Although the use of a training burst in a data sequence during initial pull-in operation may solve this problem, it is only achieved at the cost of transmission efficiency.